Nitrogen Mineralization

Nitrogen mineralization is estimated in field or laboratory experiments as the release of inorganic N from organic residues of soil organic matter. Alternatively, specific steps of the N mineralization can be estimated (e.g., arginine deaminase, urease, ammonia monooxygenase). Nitrogen availability is measured using aerobic and anaerobic incubation tests as well as soil inorganic N measurements. The recommended methods differ in incubation time and temperature, moisture content, and extraction of ammonium and nitrate. Frequently, soils are incubated under aerobic conditions and analyzed for ammonium, nitrite, and nitrate before and after incubation. Since ammonium is partly immobilized into the microbial biomass during incubation, these incubation methods yield the net production of ammonium and nitrate. Isotope pool dilution techniques enable gross rates of nitrification (or mineralization) to be determined by monitoring the decline in the 15N abundance in a nitrate or ammonium pool, labeled at t = 0, and receiving unlabeled nitrogen via nitrification or mineralization, respectively (Murphy et al., 2003). Labeled N can be applied as 15NH+ solution or injected as 15NH3 gas into soil. The use of 15N pool dilution and enrichment can also be used to separate the heterotrophic and autotrophic pathways of nitrification. An isotopic dilution experiment using 14NH415NO3 yielded rates of nitrification by the combined auto-trophic and heterotrophic paths. A parallel isotope dilution experiment with 15NH415NO3 provided the gross mineralization rate and the size and 15N abundance of the nitrate pool at different time intervals. Spatial variability of the tracer addition and extraction must be taken into account in interpreting such data.

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